Portable high rate anaerobic digester apparatus and method of operation

ABSTRACT

An apparatus for digesting waste comprises a tank, a tube, a propeller, and a trough. The tube is disposed within the tank. The propeller is disposed within the tube and configured to propel a waste material upward through the tube from a lower portion of the tank towards an upper portion of the tank. The waste material comprises heavy solids that fall to the lower portion of the tank and light solids that rise to a waste material top level located near a top of the tank. The trough is configured to collect the light solids that rise to the waste material top level. A top portion of the tube is disposed far enough below the waste material top level such that the heavy solids fall to the lower portion of the tank after being propelled up the tube rather than being propelled to at or above the waste material top level and collected by the trough.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/259,544, entitled “PORTABLE HIGH RATE ANAEROBIC DIGESTER APPARATUSAND METHOD OF OPERATION,” filed on Oct. 28, 2008, which is herebyincorporated by reference in its entirety.

BACKGROUND

Farms, vegetable processing plants, meat processing plants, creameriesand cheese processing plants, and restaurants have continually struggledwith the disposal of organic waste. For example, pig farms producetremendous amounts of pig manure. This manure has become the object ofpublic controversy because of environmental concerns such as run-offpollution and odor.

Traditionally, organic waste is disposed of by using it as fertilizer ina farming operation. However, environmental regulations now restrict thetime of the year and weather conditions during which manure, especiallyliquefied manure, can be spread; and, in some cases, require the manureto be plowed in immediately which is not always possible. This requiresthe farmer to invest in costly manure storage units or to pay to havethe manure removed. Regulations also restricts the amount of nutrients,particularly phosphorus, that can be applied in a given area of land,requiring farmers to either purchase more land or to pay other landowners to spread the excess manure.

Anaerobic digestion is another method of disposing of manure containinghigh concentrations of organic matter. Through anaerobic digestion,large quantities of organic matter are broken down by bacteria thatconvert the organic matter into biogas. Anaerobic digestion isparticularly suitable for disposing of liquefied manure containing highconcentrations of organics. In addition, operators can also use or sellthe biogas that is produced. In digested manure, nutrients arebroken-down. Consequently, plants can absorb digested manure faster,thereby requiring less land to spread a given amount of manure.

Traditional anaerobic digesters suffer from plugging where heavy andlight solids that precipitate from the organic waste coalesce intoobstructions that prevent the flow of the effluent produced duringdigestion. The plugging occurs frequently and becomes a nuisance for theoperator. Consequently, the operator (farmer, owner, etc.) devotes aconsiderable amount of time towards maintaining the anaerobic digester.Plugging of the digester interrupts the steady flow of the digestionprocess disturbing and slowing down the bacterial activity.Additionally, unplugging a digester risks the removal of the mass ofbacteria that digest the organic material. If the bacteria population isdiminished, it will take days before the bacteria repopulates to anoptimal level. Complicating matters, increasing the rate of anaerobicdigestion increases the amount of organic waste processed, whichincreases the frequency and severity of plugging.

Furthermore, traditional anaerobic digesters are typicallysemi-permanent installations and cannot be moved easily. To move adigester, a team would need to prepare a new site, disassemble thedigester, and move the individual pieces to the new site. Then, anengineering team would reassemble the digester piece-by-piece.Consequently, it is expensive to move a digester. Thus, operations thatare interested in using a digester only for a short time will bedissuaded from doing so.

For example, a crop processing plant produces a tremendous amount ofwaste during the harvest; hence, the need for a digester is great.However, during the growing season the need for digesters is diminished.Despite the periodic need, the crop processing plant does not invest ina permanent installation because of the capital requirements.

Therefore, there is a need for a high rate anaerobic digester that isportable, automated, and plugs infrequently.

SUMMARY

In a representative embodiment, an apparatus for digesting wastecomprises a tank, a tube, a propeller, and a trough. The tube isdisposed within the tank. The propeller is disposed within the tube andconfigured to propel a waste material upward through the tube from alower portion of the tank towards an upper portion of the tank. Thewaste material comprises heavy solids that fall to the lower portion ofthe tank and light solids that rise to a waste material top levellocated near a top of the tank. The trough is configured to collect thelight solids that rise to the waste material top level. A top portion ofthe tube is disposed far enough below the waste material top level suchthat the heavy solids fall to the lower portion of the tank after beingpropelled up the tube rather than being propelled to at or above thewaste material top level and collected by the trough.

In another embodiment, A digesting system comprises a plurality ofdigester devices coupled together. Each of the digester devicescomprises a tank, a tube, a propeller, a trough, and a chassis. The tubeis disposed within the tank. The propeller is disposed within the tubeand configured to propel a waste material upward through the tube from alower portion of the tank towards an upper portion of the tank. Thewaste material comprises heavy solids that fall to the lower portion ofthe tank and light solids that rise to a waste material top levellocated near a top of the tank. The trough is configured to collect thelight solids that rise to the waste material top level. The tank ismounted to the chassis. A top portion of the tube is disposed far enoughbelow the waste material top level such that the heavy solids fall tothe lower portion of the tank after being propelled up the tube ratherthan being propelled to at or above the waste material top level andcollected by the trough.

In yet another embodiment, a method of digesting waste comprisescontrolling a propeller within a tube disposed within a tank to propel awaste material upwards through the tube from a lower portion of the tanktowards an upper portion of the tank. The waste material comprises heavysolids that fall to the lower portion of the tank and light solids thatrise to a waste material top level located near a top of the tank. Themethod further comprises collecting the light solids that rise to thewaste material top level in a trough. A top portion of the tube isdisposed far enough below the waste material top such that the heavysolids fall to the lower portion of the tank after being propelled upthe tube rather than being propelled to at or above the waste materialtop level and collected in the trough.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representative block diagram of a digester system inaccordance with a representative embodiment.

FIG. 2 is a side view of a digester apparatus in the digester system ofFIG. 1 in accordance with a representative embodiment

FIG. 3 is a flowchart of operations performed by a digester apparatus inaccordance with a representative embodiment.

FIG. 4 is a front view of a digester apparatus mounted on a chassis inaccordance with a representative embodiment.

DETAILED DESCRIPTION

A portable high rate anaerobic digester apparatus and method ofoperating are described. In the following description, for purposes ofexplanation, numerous specific details are set forth to provide athorough understanding of exemplary embodiments of the invention. Itwill be evident, however, to one skilled in the art that the inventionmay be practiced without these specific details. The same number is usedin the figures to represent the same element. The figures are not toscale. In other instances, well-known structures and devices are shownin simplified form to facilitate description of the representativeembodiments.

Structure of the Digester Apparatus

Referring to FIG. 1, a front view of a digester apparatus 100 inaccordance with a representative embodiment is shown. The digesterapparatus 100 includes a digester tank 101. The digester tank 101includes a fluid inlet 102, a heavy solids outlet 103, a drainage trough104, a vertical pump tube 105, a maintenance porthole 109, a transferoutlet 107, and a biogas outlet 140. The digester tank 101 is made outof steel; however, stainless steel, polyethylene, concrete, or othermaterials commonly used in vat construction can be used. The maintenanceporthole 109 is used to access the digester tank 101 for maintenancepurposes. The transfer outlet 107 is used to drain the digester tank 101or transfer partially digested waste to another tank. The digester tank101 also includes sampling ports and additional maintenance ports.

In a representative embodiment, the digester tank 101 is approximately40-50 feet tall and 12-14 feet wide. The digester tank 101 holds30,000-40,000 gallons. The ratio of the side walls to the diameter ofthe digester tank 101 is preferably 2.5-4.5. The bottom of the digestertank 101 has a conical shape.

The digester tank 101 is heated by a hot-water jacketed cone at thebottom of the digester tank 101 (not shown) supplied by a boiler (notshown). Placing heating elements outside of the tank eliminates internalobstructions which promote plugging. Alternatively, the digester tank101 is heated by resistance-type heating elements wrapped around thebody of the digester tank 101. Heating can also be accomplished withfluid-based heat exchangers, circulating glycol or water heated by awaste gas burner, disposed as coils inside the digester tank 101.

The digester tank 101 is insulated (not shown). The insulation can be ainsulating blanket, for example, made of fiberglass. Alternatively, thedigester tank 101 is double-walled and the cavity between the walls isfilled with a polyurethane or isocyanate-type expanding foam.

During operation, the digester tank 101 is filled with an organic wasteeffluent such as manure. Bacteria, either added to the waste ornaturally occurring in the waste, convert the manure into a number ofproducts including biogas, water, light solids (specific gravity lessthan 1), and heavy solids (specific gravity greater than 1). The biogasdegasses from the waste. The biogas produced is primarily methane (alongwith carbon dioxide and hydrogen sulphide) although designer bacteriacan be introduced that produce other hydrocarbons. The light solids aretypically animal bedding such as straw, sawdust, and other undigestedanimal food particles. The light solids rise to the top of the waste andheavy solids fall to the bottom of the waste.

The digester tank 101 is air-tight. The biogas exits the digester tank101 through a biogas outlet 140. The biogas outlet 140 leads to a gasprocessing apparatus (not shown) that prevents backflow and regulatespressure. Biogas needs to be treated to remove water and othernon-methane gases before it can be piped into the gas lines. The gas isthen either used in powering the digester apparatus 100, piped to otheron-site operations, or refined and distributed. For example, thedigester apparatus 100 can be connected to a gas pipeline network.

The fluid inlet 102 is located on the side wall of the digester tank 101underneath a digester tank waste material top level 190. The fluid inlet102 is connected to a pump 111 and a heater 115. In a representativeembodiment, waste material is pre-heated by heater 115 and pumped bypump 111 into the digester tank 101. The heater 115 may be after thepump 111 or may be a combination of both before and after the pump 111.In one embodiment, waste material is pumped into the digester tank 101at a constant rate. Alternatively, waste material is pumped into thedigester tank 101 on demand. The amount of waste pumped into thedigester is measurable and controllable in order to control thehydraulic retention time.

The heavy solids outlet 103 is located at the bottom of the digestertank 101. The heavy solids outlet 103 is connected to the bottom of awaste column 125. The waste column 125 is a vertical cylinder such as apipe. The waste column 125 is arranged as a double valve chamber where afirst waste valve 121 separates the heavy solids outlet 103 and thewaste column 125, and a second waste valve 127 separates the wastecolumn 125 from a first lagoon outlet 129. The first lagoon outlet 129is connected to a hose or pipe that empties into a lagoon or holdingpond. Alternatively, the first lagoon outlet 129 can lead to a holdingtank which is emptied periodically. The waste column 125 should be atleast a height 193 that is one gas trap submersion height 195 above thedigester tank waste material top level 190 so that heavy solids do notspill out of the waste column 125. The first waste valve 121 and thesecond waste valve 127 usually remain closed. Alternatively, the wastecolumn 125, the heavy solids outlet 103, and/or the first lagoon outlet129 have augers which can be used to move the heavy solids.

When enough heavy solids have collected at the bottom of the digestertank 101, the first waste valve 121 opens while the second waste valve127 is closed. Hydraulic pressure forces the heavy solids into the wastecolumn 125. The first waste valve 121 closes and the heavy solids arenow trapped in the waste column 125. After the waste column 125 is full,the second waste valve 127 is opened while the first waste valve 121remains closed. The heavy solids are forced out of the first lagoonoutlet 129 and into a holding pond.

Referring now to FIG. 2, a side view of the digester apparatus 100 ofFIG. 1 in accordance with a representative embodiment is shown. Thedrainage trough 104 is located near the top of the digester tank 101 sothat the top of the drainage trough 104 corresponds with the digestertank waste material top level 190. The drainage trough 104 is horizontaland offset from the center of the digester tank 101 so that a main driveshaft 151 can pass through the centerline of the digester tank 101. Thedrainage trough 104 is U-shaped. Hence, when light solids rise to thetop of the waste, which corresponds to the digester tank waste materialtop level 190, the light solids fall into the drainage trough 104.Likewise, when the level of the waste rises past the digester tank wastematerial top level 190, the effluent at the top of the digested wastedrains into the drainage trough 104. The portion of the drainage trough104 that extends beyond the digester tank 101 is air-tight so that thebiogas cannot escape.

Referring again to FIG. 1, an auger 160 runs across or near the centerof the drainage trough 104. The auger 160 is driven by an auger motor165. The auger 160 usually runs at a constant speed; however, the augercan be programmed to run periodically. When light solids fall into thedrainage trough 104, the turning auger 160 pushes the light solids tothe end of the drainage trough 104. Additional augers can be placedthroughout the digester apparatus 100 wherever plugging is problematic.

At the end of the drainage trough 104 the light solids and effluent ofdigested waste fall into a drainage tube 131. The drainage tube 131 isconnected to gas trap tank 130 where the light solids and effluent ofdigested waste are stored temporarily. The discharge end of the drainagetube 131 is submerged one gas trap height 195 into the liquid of the gastrap tank 130 in order to create a desirable operating pressure in thedigester tank 101. The effluent of the gas trap tank 130 is drained offthe top of the gas trap tank 130 through a second lagoon outlet 137. Thesecond lagoon outlet 137 is connected to a hose or pipe that emptiesinto a lagoon or holding pond. Alternatively, the second lagoon outlet137 is connected to a holding tank that is emptied periodically. Thedrainage tube 131 can also have its own auger which prevents plugging.

The gas trap tank 130 is equipped with a mixer 133. The mixer 133 isdriven by a mixer motor 135. The mixer 133 is used to agitate the lightsolids and effluent of digested waste whenever more waste is pumped intothe digester tank 101.

The digester tank 101 includes an agitation and skimming system in orderto promote digestion and prevent clogging. The vertical pump tube 105 isa tube of steel or stainless steel positioned vertically in the centerof the digester tank 101 by fins (not shown) that are welded to theinside of the digester tank 101. Hence, organic waste and biomass canflow on the outside and inside of the vertical pump tube 105.

A propeller 157 is located inside of the vertical pump tube 105. Thepropeller 157 is connected to the main drive shaft 151 driven by a mainmotor 150. When propeller 157 spins, the waste material is pumped upwardthrough the vertical pump tube 105 and allowed to return downward aroundthe outer walls of the digester tank 101. Hence, a convection-likecurrent is created. The recycling movement of the organic wastemaintains the heavier solids in suspension and improves bacteria contactwith the organic waste material. Additionally, the propeller 157 can bedesigned to pulverize and mix the organic waste material as it passesthrough the vertical pump tube 105.

A skimmer motor 158 drives a skimmer drive shaft 152. The skimmer driveshaft 152 and the main drive shaft 151 enter the digester tank 101through a sealed coupling 153. The skimmer drive shaft 152 is concentricwith the main drive shaft 151. The main drive shaft 151 typically spinsmuch faster than the skimmer drive shaft 152. The sealed coupling 153also prevents biogas from escaping the digester tank 101. The skimmermotor 158 and the main motor 150 can be run independently and atdifferent, varying speeds. Alternatively, the main drive shaft 151 andthe skimmer drive shaft 152 can be driven by the same motor; andrelative angular velocity can be modified using a differential.

The skimmer drive shaft 152 drives a skimmer 155. The skimmer 155 is anarm or arms that skim the light solids off the top of the waste and intothe drainage trough 104. In a representative embodiment, the arms havebrushes attached so that the skimmer 155 can reach below the digestertank waste material top level 190 while the skimmer 155 rotates.However, the skimmer 155 is adjustable and can be positioned at variouslevels relative to the digester tank waste material top level 190 andthe top of the drainage trough 104. Alternatively, the skimmer 155 canhave a plain face or have a profile like a rake. The skimmer can becontrolled to operate in either clockwise, counterclockwise, oralternating rotational direction to improve the removal of the lightersolids.

The digester apparatus 100 is automatically controlled by a controller180. The controller 180 includes a programmable logic controller (PLC),motor contacts, motor controllers, relays, sensor inputs, outputs, and ahuman machine interface. The PLC operates the first waste valve 121, thesecond waste valve 127, the heater 115, the pump 111, the main motor150, the skimmer motor 158, the auger motor 165, and the mixer motor135. The PLC also controls the hot-water jacketed cone used to heat thedigester tank 101. The PLC also monitors the state of the digesterapparatus 100 using temperature sensors, pressure sensors, encoders, andproximity sensors. In particular, the digester apparatus 100 includes avariety of sensors that test the waste itself. Consequently, thecontroller can determine when the waste has produced the optimum yieldof biogas, for example, by determining the temperature, pH, specificgravity, ratio of substrates, and hydraulic flowrate of the organicwaste and digested effluent, or the amount of gas produced. A user canalso override the automatic controls. The user can also remotely monitorand control the system. Alternatively, the controller 180 can be anycomputer or circuit.

Referring to FIG. 3, a front view of a digester apparatus 300 mounted ona chassis 310 in accordance with a representative embodiment is shown.The digester apparatus 300 is mounted on the steel chassis 310 so thatthe digester apparatus 300 can be easily constructed at a factory andmoved to a operations site. Hence, the digester apparatus 300 ispackaged as a self-contained module or modules. Smaller versions of thedigester apparatus 300 mounted on the chassis 310 can be moved by acrane and a flatbed truck using lifting points 330. Smaller versions ofthe digester apparatus 300 can be moved by a forklift and a trailerusing skid holes 320.

Likewise, digester apparatus 300 can be easily broken into modules andsplit between a number of chassis. In a representative embodiment, adigester tank assembly comprising a digester tank and a waste column is50 feet tall, including legs, and 14 feet by 14 feet wide. A separatemodule comprising a gas trap tank, a controller, pumps, heaters, andboilers is mounted on the steel chassis 310. The digester tank assemblyand module are shipped to a site on separate flatbed trailers. Thedigester tank assembly is removed and positioned upright. The module isplaced in proximity of the digester tank assembly. The installation teamsimply connects power, control wiring, and piping for the digester tankassembly and the module.

Hence, the digester apparatus 300 can be easily moved from site to sitewith minimal effort and without the support of an engineering team. Theportability of digester apparatus 300 makes it easy for a factory orfarm to rent a digester apparatus based on periodic need rather thaninvest in a permanent installation. Farmers, food processing plants etc.could also purchase the packaged digester through financing, using thedigester as collateral. The digester would not become part of the realestate; it would be financed either through a bank or leasing company asa separate piece of equipment.

Additionally, a plurality of self-contained modules can be gangedtogether where a large amount of waste needs to be processed. A masterself-contained module can direct the activity of the plurality ofself-contained modules. For example, the master self-contained modulecan control the overall waste intake and direct which slave modulereceives new waste in order to prevent a bacteria mass from dying off.

In another embodiment, the gang of self-contained modules can also beplaced in series where the output of one module is the input to the nextmodule. Each self-contained module has a different kind of bacteria thatperforms a specific function, thereby increasing the efficiency andyield of the gang of modules. For example, bacteria X in module X worksparticularly well when the waste it digests is in a state Y. Bacteria Win module W produces a significant amount of biogas and a waste productin state Y. Hence the output of module W is fed into module X therebyincreasing efficiency and yield of the overall system. Partiallydigested waste can be transferred out of a digester tank through atransfer outlet. Alternatively, waste captured by a gas trap tank can beused as input for another module.

More particularly, separate self-contained modules can be devotedtowards specific anaerobic digestion stages, namely: hydrolysis,acidogenesis and methanogenesis. In another embodiment, a firstself-contained module specializes in hydrolysis and acidogenesis. Asecond self-contained module takes the output of the firstself-contained module and specializes in the methanogenesis stage.

Operation and Control of the Digester Apparatus

Referring to FIG. 4, a flowchart of the operation of a digesterapparatus in accordance with a representative embodiment is shown. Thedigester apparatus is automatically controlled by a controller. Thecontroller performs the operations based on the ability of the digesterapparatus to accept more waste, and in no particular sequence. In arepresentative embodiment, the waste is dairy cow manure. However, anyorganic waste can serve as feedstock for the digester such as farmanimal manure, waste products from vegetable processing plants, meatprocessing plants, creameries and cheese processing plants, restaurantcooking oil, grease waste, etc.

In a check fluid level operation 410, the controller determines if thedigester tank is full by reading a fluid proximity sensor located in thedigester tank. Alternatively, the fluid level in the tank can bedetermined using a float and switch. If the digester tank is not full,the controller tries to fill the digester tank with more waste.

In a fill digester tank operation 415, the controller first determinesif there is feed waste to fill the digester tank with. If there is wasteavailable, the controller activates the heater. Once the feed waste isat an acceptable temperature, the controller activates the pump. Thedigester apparatus can operate at either mesophilic (95-98° F.) orthermophilic (140-145° temperature range. The pump moves the heatedwaste into the digester tank. The controller also takes into accountthat the gas trap tank must be charged with effluent (raw organicwaste). The controller also activates the mixer in the gas trap wheneverthe tank is charged with organic waste in order to assimilate overflowfrom the digester tank caused by additional filling.

After the digester tank is filled, the heater and pump continue tosupply heated waste to the digester tank at a controlled feed rate. Feedrates can vary in order to obtain a hydraulic retention time from 5 to20 days based on the quality of the waste and the mass of bacteriaavailable to digest. The controller can dynamically alter the retentiontime based on how quickly the waste is digested.

The amount organic waste pumped into the digester relates to thedigester retention time: (Digester Volume)/(Volume/Day)=Retention Timein Days. As raw manure is pumped into the digester, the waste level willrise and the digested effluent will flow into the drainage troughthereby maintaining the desired operating level. A float, proximityswitch, ultrasonic sensor, or other kind of fluid level switch can sensean over fill situation that indicates a plug.

As the waste digests, heavy solids that have a specific gravity higherthan water fall out of suspension and accumulate in the cone at thebottom of the digester tank. After a sufficient amount of heavy solidshave accumulated, in a heavy solids purge operation 420, the controllermoves the heavy solids to the waste column. The heavy solids purgeoperation 420 is designed to retain as much active bacteria as possible,thereby reducing the hydraulic retention time. As described above, thewaste column is arranged as a double valve chamber.

During normal operation, the first waste valve and the second wastevalve of the waste column remain closed. When controller determines thatenough heavy solids have accumulated, through time and measurement, thecontroller opens the first waste valve. The accumulated heavy solids areforced up into the waste column through the hydraulic force of the wastematerial in the digester tank. The controller closes the first wastevalve after the fluid levels in the waste column and digester tank arein hydraulic equilibrium. It may be desirable to leave some heavy solidsin the bottom of the digester tank in order to maintain a seal that willprevent effluent from leaving the digester tank. Alternatively, thecontroller may use a number of iterations to fill the waste column.

After the waste column is full, the controller opens the second wastevalve. The captured heavy solids are forced out of a first lagoon outletand into a lagoon or holding pond. Alternatively, the first lagoonoutlet can lead to a holding tank which is emptied periodically.Afterward, the controller closes the second waste valve.

In a recycle operation 425, the controller activates the propeller. Thecontroller runs the main motor constantly or periodically. When thepropeller spins, the organic waste material is pumped upward through thevertical pump tube and allowed to return downward around the outer wallsof the digester tank. The controller also varies the pump speed toobtain the optimum internal hydraulic velocity in order to maximize theamount of solids in suspension. The recycling movement of the organicwaste maintains the heavier solids in suspension and improves bacteriacontact with the organic waste material. Lighter, fibrous materialfloats to the top of the digester tank.

In another embodiment, the propeller can be programmed to createdifferent suspension profiles. For example, the blades of the propellercan be programmed to act similarly to the agitator of a washing machine.Hence, various agitation cycles can be created to handle different typesand qualities of waste.

In a skim operation 430, the controller activates the skimmer and theauger. The accumulation of the lighter, fibrous material is removed by askimmer with two rotating sweep arms. The skimmer moves over the top ofand against the floating solids thereby pushing the solids into thedischarge trough. The skimmer can be rotated at various speeds and indifferent directions. In a representative embodiment, the skimmer isrotated in a first direction alternating with a second direction. Inanother embodiment, the skimmer is rotated for an extend period of timein one direction and periodically reversed to knock accumulated wasteoff of the skimmer. In another embodiment, the skimmer is programmed toflick accumulated waste off of the skimmer by quickly running theskimmer blades back and forth over the edge of the drainage trough.

In the skim operation 430, the controller also activates the augermotor. Whenever the skimmer is running, the auger should also run inorder to prevent plugging of the discharge trough. As the solids enterthe discharge trough, the auger pulls the solids towards the dischargetube where the solids fall into the gas trap tank. The auger can bereversed to un-jam plugs.

Advantageously, the check fluid level operation 410, the fill digestertank operation 415, the heavy solids purge operation 420, the recycleoperation 425, and the skim operation 430 work together to increase therate of anaerobic digestion and keep the digester apparatus unplugged.Additionally, the digester apparatus processes organic wasteautomatically at a high rate and plugs infrequently. Additionally, thedigester apparatus can be easily manufactured at a factory and then bemoved easily from site to site.

The foregoing description of representative embodiments has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the present invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of thepresent invention. The embodiments were chosen and described in order toexplain the principles of the present invention and its practicalapplication to enable one skilled in the art to utilize the presentinvention in various embodiments and with various modifications as aresuited to the particular use contemplated.

What is claimed is:
 1. An apparatus for digesting waste, comprising: atank; a tube disposed within the tank; a propeller disposed within thetube and configured to propel a waste material upward through the tubefrom a lower portion of the tank towards an upper portion of the tank,wherein the waste material comprises heavy solids that fall to the lowerportion of the tank and light solids that rise to a waste material toplevel located near a top of the tank; and a trough configured to collectthe light solids that rise to the waste material top level; wherein atop portion of the tube is disposed far enough below the waste materialtop level such that the heavy solids fall to the lower portion of thetank after being propelled up the tube rather than being propelled to ator above the waste material top level and collected by the trough. 2.The apparatus of claim 1, further comprising: a controller configured tooperate the propeller to maintain at least a portion of the heavy solidsin suspension within the waste material in the tank during at least partof a recycle operation.
 3. The apparatus of claim 1, further comprising:an auger disposed inside the trough and configured to pull the lightsolids collected by the trough toward a discharge tube of the tank; anda skimmer disposed at an edge of the drainage trough and configured topush the light solids over the edge of the trough into the trough. 4.The apparatus of claim 3, wherein the skimmer comprises at least one ofa brush or a blade.
 5. The apparatus of claim 1, further comprising: anoutlet tube located at the lower portion of the tank through which theheavy solids are purged from the tank.
 6. The apparatus of claim 5,further comprising: a double valve chamber configured to collect theheavy solids purged from the tank, the double valve chamber including: afirst valve; a second valve; and a column; wherein the first valve andthe second valve are located at a lower portion of the column.
 7. Theapparatus of claim 6, further comprising a controller configured tooperate the double valve chamber.
 8. The apparatus of claim 6, furthercomprising a second tank configured to store discharge from the trough.9. The apparatus of claim 1, wherein the trough is offset from a centerof the tank and a drive shaft configured to drive the propeller ispositioned vertically along the center of the tank.
 10. The apparatus ofclaim 1, further comprising a chassis, wherein the tank is mounted onthe chassis and the apparatus is movable between locations.
 11. Theapparatus of claim 1, wherein the digester tank has a height to widthratio between 2.5 and 4.5.
 12. The apparatus of claim 1, wherein thetrough is U-shaped and is positioned horizontally, wherein an edge ofthe trough is positioned at the waste material top level.
 13. Adigesting system comprising: a plurality of digester devices coupledtogether, each of the digester devices comprising: a tank; a tubedisposed within the tank; a propeller disposed within the tube andconfigured to propel a waste material upward through the tube from alower portion of the tank towards an upper portion of the tank, whereinthe waste material comprises heavy solids that fall to the lower portionof the tank and light solids that rise to a waste material top levellocated near a top of the tank; a trough configured to collect the lightsolids that rise to the waste material top level; and a chassis, whereinthe tank is mounted to the chassis; wherein a top portion of the tube isdisposed far enough below the waste material top level such that theheavy solids fall to the lower portion of the tank after being propelledup the tube rather than being propelled to at or above the wastematerial top level and collected by the trough.
 14. The digesting systemof claim 13, wherein the digester devices are coupled together in seriessuch that an output of a first digester device is an input of a seconddigester device.
 15. The digesting system of claim 14, wherein the firstdigester devices is configured to perform hydrolysis and acidogenesisand the second digester devices is configured to performmethanogenesis.16. The digesting system of claim 13, wherein the digester devices arecoupled together in parallel such that a common input feeds into each ofthe plurality of digester devices and an output of each of the pluralityof digester devices feeds into a common output.
 17. A method ofdigesting waste, the method comprising: controlling a propeller within atube disposed within a tank to propel a waste material upwards throughthe tube from a lower portion of the tank towards an upper portion ofthe tank, wherein the waste material comprises heavy solids that fall tothe lower portion of the tank and light solids that rise to a wastematerial top level located near a top of the tank; and collecting thelight solids that rise to the waste material top level in a trough,wherein a top portion of the tube is disposed far enough below the wastematerial top such that the heavy solids fall to the lower portion of thetank after being propelled up the tube rather than being propelled to ator above the waste material top level and collected in the trough. 18.The method of claim 17, further comprising discharging the light solidscollected in the trough through a discharge tube.
 19. The method ofclaim 17, further comprising purging the heavy solids from the tankthrough an outlet tube located at the lower portion of the tank.
 20. Themethod of claim 19, further comprising controlling a double valvechamber to collect the heavy solids purged from the tank.